Spatial variability and temporal changes of drought generation processes over Europe and the Alps

Hydrological extreme events are generated by different sequences of hydro-meteorological drivers, the importance of which may vary within the sample of drought events and in space and time. Here, we investigate how the importance of different hydro-meteorological driver sequences varies by event magnitude, in space, and in time using large samples of catchments in Europe and the Alps. To do so, we develop an automated classification scheme for streamflow drought events, which assigns events to one of eight drought event types - each characterized by a set of single or compounding drivers. Our results show that (1) moderate droughts are mainly driven by rainfall deficits while severe events are mainly driven by snowmelt deficits; (2) rainfall deficit droughts and cold snow season droughts are the dominant drought event type in Western Europe and in Eastern and Northern Europe, respectively; (3) temporal changes in both drought intensity, deficit, and duration and generation processes are stronger in high- than in low-elevation catchments; and (4) in high-elevation catchments, snowmelt-deficit-induced droughts become more frequent, leading to increases in drought deficits. We conclude that climate impact assessments on droughts can profit from assessing changes in drought generation processes to improve the understanding of how drought magnitudes are changing in a warming world.